1. Field of the Invention
The present invention relates to a copying machine, a printer and other image forming apparatuses which use an electrophotographic system or an electrostatic recording system, particularly to an image forming apparatus which is provided with a plurality of image forming sections and which can form an image in a plurality of colors.
2. Related Background Art
In recent years, as an electrophotographic apparatus, a color image forming apparatus of a tandem system has been developed which has image bearing members (photosensitive members) and developing units for four colors and which forms a color image by overlapping four-color images on a sheet with one path. The apparatus has a merit suitable for performing color recording at a high speed.
Even in this system, image formation can also be performed with a single color, and in this case an image forming process of a color with which no image is formed is stopped. However, since the photosensitive member lightly contacts a belt for conveying a sheet or a resin sheet as a transfer material, the photosensitive member is usually rotated not to be damaged.
On the other hand, the miniaturization of the image forming apparatus has been advanced, but even when each of the charging, exposing, developing, transferring, fixing, and cleaning processes is reduced in size, there is a limitation. Moreover, after the transferring process, the transfer residual toner remaining on the surface of the photosensitive member is cleaned by a cleaner and collected (recovered) as waste toner, but it is preferable not to generate this waste toner.
To solve the problem, a cleanerless image forming apparatus has been proposed in which developing and cleaning are performed by a developing unit at the same time. This simultaneous developing and cleaning (cleaning simultaneous with developing) method comprises recovering the transfer residual toner on the photosensitive member by a fog removing bias into the developing unit during developing in the subsequent processes. According to this method, since the transfer residual toner can be used in the subsequent processes, the waste toner can be eliminated, and the trouble of maintenance can be saved. Furthermore, advantages are also large in a spatial respect, and the image forming apparatus can remarkably be miniaturized.
When this cleaner system is employed in the image forming apparatus of the tandem system, a new problem has arisen.
Specifically, during the monochromatic mode in the tandem system, or when originals with images of single color and a plurality of colors are mixed and loaded, the sheet as the transfer material or a second image bearing member contacts each color photosensitive member, and the photosensitive member of the color with which no image is formed has to be simultaneously rotated. Moreover, particularly when the image forming section of the color with which the image is to be formed is not disposed first in order of passage of the transfer material, a transfer bias for adsorbing the transfer material to a conveying belt has to be applied in the image forming section of the color with which no image is formed. In this case, while toner is still unconsumed, the photosensitive member surface is influenced by paper dust or electric discharge by transfer. Therefore, a smeared image is remarkably generated.
When a cleaning blade is disposed, the surface of the photosensitive member is abraded with the toner or an externally added material by the abutment pressure of the blade. In the cleanerless system, however, there is a problem of the smeared image (smudging) during the full-color image formation subsequent to the monochromatic mode.